Harmon quickly jumped aboard 'West Wing' train

LOS ANGELES - It didn't take Mark Harmon long to accept the season-ending, four-episode guest role he has on The West Wing.

"I got a scene faxed to me by my agent late on a Wednesday afternoon, and Thursday morning I was working," Harmon says.

The former St. Elsewhere and Chicago Hope star made his first appearance on NBC's White House drama Wednesday, playing a Secret Service agent assigned to protect press secretary C.J. Cregg (Allison Janney) after she receives a death threat.

Harmon, who's worked with David E. Kelley and Tom Fontana on past series - he's also directed a few episodes of Kelley's Boston Public - says the quality of the acting and writing on The West Wing is what led him to take the role.

"I appreciate that I'm in very rare air here," says Harmon, who counts himself among the fans of The West Wing's creator, Aaron Sorkin. "So often the formula [of a series] gets changed by outside pressures, but they've been able [to create an atmosphere] where it's all about the work. This train called The West Wing is moving fast, and you're expected to keep up."

That was a daunting task in Harmon's first few days on the set, but he says he's now used to the rhythm of the show. "Where the show is right now, we have pages coming down daily," he says. "We just started the season finale, and yet there are still scenes from [the next-to-last episode] that I have left to shoot."

There was immediate tension between Harmon's character, Donovan, and C.J., but the actor is reluctant to say whether it will turn into the romantic variety.

"[The characters] are two very smart people who are used to doing things their own way," he says. As for the chemistry between the two, "Part of that is from the writing, and part of it is just the pleasure of playing opposite Allison. She's a gifted actress, and it's just fun to bat it back and forth."